1. Field of Invention
The present invention relates to a blood-pressure monitor apparatus which monitors the chance of blood pressure of a living subject, based on blood-pressure-relating information which changes in relation with the change of blood pressure of the subject.
2. Related Art Statement
As a pulse-wave-propagation-velocity ("PWPV") relating information which relates to a velocity at which a pulse wave propagates through an artery of a living subject, there is known a propagation time, DT, in which the pulse wave propagates between two different portions of the artery, or the propagation velocity, V.sub.M, (m/s) itself. It is known that the PWPV-relating information has, within a certain range, a substantially proportional relationship with the blood pressure, BP, (mmHg) of the living subject. Hence, there has been proposed a blood-pressure ("BP") monitor apparatus which determines, in advance, unknown coefficients, .alpha. and .beta., of a linear expression, EBP=.alpha.(DT)+.beta. (.alpha. is a negative constant), or EBP=.alpha.(V.sub.C)+.beta. (.alpha. is a positive constant), based on subject's BP value, BP, and a piece of PWPV-relating information (DT or V.sub.M) both of which are measured in advance, and then determines, according to the thus determined linear expression, an estimated BP value, EBP, of the subject based on each one of pieces of PWPV-relating information iteratively obtained after the last BP measurement using a cuff. Thus, the BP monitor apparatus can monitor the blood pressure of the subject. If one of the estimated BP values EBP iteratively determined after the last BP measurement has largely deviated from the estimated BP value at the time of the last BP measurement, the BP monitor apparatus starts a BP measurement using the cuff.
Meanwhile, it is known that the blood pressure of a living subject is regulated by the cardiac output (1/min), and the peripheral vascular resistance, of the subject. More specifically described, as the cardiac output increases, the blood pressure increases; and as the cardiac output decreases, the blood pressure decreases. As the peripheral vascular resistance increases, that is, the peripheral blood vessels contract, the blood pressure increases; and as the peripheral vascular resistance decreases, that is, the peripheral blood vessels expand, the blood pressure decreases. The cardiac output is obtained as the product of stroke volume (i.e., volume of blood Output from the heart per stroke or beat) and heart rate. Therefore, heart-rate-relating information which relates to heart rate, such as heart (pulse) rate itself, or pulse period, changes with the change of the blood pressure. In addition, as the peripheral vascular resistance increases, i.e., the peripheral blood vessels contract, the area enveloped by the waveform of each of heartbeat-synchronous pulses of a peripheral pulse wave detected from subject's peripheral portion, decreases. Thus, the area of each pulse of the peripheral pulse wave is influenced by the change of the peripheral vascular resistance. That is, the area of each pulse of the peripheral pulse wave changes with the change of the blood pressure. These phenomena are utilized by another BP monitor apparatus which starts a BP measurement using a cuff if one of pieces of pulse-rate-relating information iteratively obtained, or one of pulse areas iteratively obtained, after the last BP measurement using the cuff has deviated by not less than a predetermined amount from the piece of pulse-rate-relating information, or the pulse area, at the time of the last BP measurement. An example of this BP monitor apparatus is disclosed in Japanese Patent Publication No. 10-43147 and the corresponding U.S. Pat. No. 5,752,920.
However, the conventional BP monitor apparatuses do not display any piece of BP-relating information, or display only the current one of pieces of BP-relating information iteratively obtained after the last BP measurement using the cuff. Therefore, a person such as the subject as a patient or a medical staff who attends the patient cannot judge, from the screen image displayed by each conventional monitor apparatus, that the patient's current condition is not at a critical level which needs a BP measurement using the cuff, but is near to that level, or that the patient's current condition has not changed so much from his or her condition at the time of the last BP measurement. In addition, regarding the conventional BP monitor apparatus which displays only the current one of pieces of BP-relating information iteratively obtained after the last BP measurement, it may be difficult for a person to select an appropriate threshold value, relative to the piece of BP-relating information at the time of the last BP measurement, that is used in judging whether the current piece of BP-relating information is abnormal, that is, whether a BP measurement using the cuff is needed.
In addition, the relationship between subject's blood pressure and PWPV-relating information changes because it is influenced by the condition of subject's central body portion, such as the condition of cardiac muscle, and the condition of subject's peripheral body portion, such as the hardness of peripheral blood vessels or the resistance to blood flows. Therefore, in the BP monitor apparatus disclosed in the above-indicated Japanese Patent Publication No. 10-43147 (and the U.S. Pat. No. 5,752,920), the heart-rate-relating information is used as the central-side information, the area of each pulse of the peripheral pulse wave is used as the peripheral-side information and, if the current PWPV-relating information (or the current estimated BP value determined based on the current PWPV-relating information) has changed by not less than a predetermined amount from the PWPV-relating information (or the estimated BP value) at the time of the last BP measurement and simultaneously if the heart-rate-relating information and/or the area of each pulse of the peripheral pulse wave has changed by not less than a predetermined amount from the heart-rate-relating information and/or the area of one pulse of the peripheral pulse wave at the time of the last BP measurement, a BP measurement using the cuff is started.
However, in the above conventional BP monitor apparatus which uses plural sorts of BP-relating information in judging whether a BP measurement should be started, no pieces of BP-relating information of each sort are displayed, or only the current one of pieces of BP-relating information of each sort, iteratively obtained after the last BP measurement, or a time-wise change of the pieces of BP-relating information of each sort is displayed. Therefore, it is more difficult for a person to judge, from the screen image displayed by the BP monitor apparatus, that the patient's current condition is not at a critical level which needs a BP measurement using the cuff, but is near to that level, or that the patient's current condition has not changed so much from his or her condition at the time of the last BP measurement, than to judge, based on a single sort of BP-relating information, whether a BP measurement should be started. In addition, even when the BP monitor apparatus may start a BP measurement using the cuff, the display device of the apparatus informs only that the abnormality of BP-relating information has occurred, and does not inform which sort of BP-relating information has become abnormal. For example, in the case of the BP monitor apparatus which starts a BP measurement using the cuff if the current PWPV-relating information has changed by not less than a predetermined amount from the PWPV-relating information at the time of the last BP measurement and simultaneously if the heart-rate-relating information and/or the area of each pulse of the peripheral pulse wave has changed by not less than a predetermined amount from the heart-rate-relating information and/or the area of one pulse of the peripheral pulse wave at the time of the last BP measurement, a person cannot judge whether the PWPV-relating information and the heart-rate-relating information have largely changed, whether the PWPV-relating information and the area of each pulse of the peripheral pulse wave have largely changed, or whether all of the PWPV-relating information, the heart-rate-relating information, and the area of each pulse of the peripheral pulse wave have largely changed.
Moreover, in the above conventional BP monitor apparatus, since plural sorts of BP-relating information are employed in judging whether a BP measurement should be started, a person has a high degree of freedom in selecting respective threshold values which are used in judging whether a BP measurement should be started. Thus, it is more difficult for a person to select, for each of the plural sorts of BP-relating information, an appropriate threshold value, relative to the piece of BP-relating information of the each sort at the time of the last BP measurement, that is used in judging whether a BP measurement using the cuff should be started, than to select, for a single sort of BP-relating information, an appropriate threshold value to be used in judging whether a BP measurement should be started.